1. Field of the Invention
The present invention relates to a clad bonding wire for connecting the electrodes of a semiconductor device to external leads and, more particularly, to a clad bonding wire suitable for connecting the electrodes of a semiconductor device to external leads by a ball bonding method.
2. Description of the Prior Art
In electrically connecting a semiconductor device to external leads with a bonding wire, such as a Au wire, by a ball bonding method, the tip of a bonding wire projecting from a capillary is fused in a ball with an electric torch, and the molten ball is pressed against the bonding pad of the semiconductor device to connect the bonding wire to the bonding pad, the bonding wire is extended in a curve between the bonding pad and an external lead, the bonding wire is connected to the external lead, and then the bonding wire is cut.
When the tip of the conventional bonding wire is heated to form a ball. a neck having a mechanical strength lower than that of the bonding wire is formed behind the ball. Therefore, the neck is liable to be broken during the wire bonding operation, the bonding wire making an electrical connection droops or bends due to the yielding of the neck, and the neck is liable to be broken by the concentration of stress on the neck due to alternate thermal expansion and thermal contraction caused by by cyclic temperature variation during temperature cycling test.
On the other hand, the recent progressive increase in the packaging density of LSIs and resultant increase in the number of pins on each LSI package require the use of finer bonding wires and the reduction of bonding pitch. However, reduction in diameter of the conventional bonding wire is limited and hence the conventional bonding wire is unable to meet the foregoing requirement because the neck formed in the known bonding wire is liable to break.
A composite bonding wire formed by cladding a core wire of high-purity Ag with high-purity Au or an Au alloy has been proposed in Japanese Patent Laid-open (Kokai) No. Sho 56-21354 to solve problems in the conventional bonding wires. However, this composite bonding wire has the following drawbacks. In a composite bonding wire formed by cladding a Ag core wire with Au cladding, a diffused layer of a sufficient thickness cannot be formed between the core wire and the cladding in the neck when the composite wire is heated to form a ball if the ratio of the diameter of the core wire to the outside diameter of the cladding is in a certain range, and hence the strength of the neck cannot be increased or even if a diffused layer of a sufficient thickness is formed in the neck, an excessively hard ball is formed to crack the package when the ball is pressed against the bonding pad of the package.